Abstract Compositional variability found in modern mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) reflects differentiation processes associated with material recycling in the mantle-crust system. To investigate the timing at which this recycling system was established and how it transformed into the present-day system, we present geochemical analyses of the Archean basalts from North Pole (NP) in the East Pilbara Craton, Western Australia, and the Isua supracrustal belt (ISB), southern West Greenland. These rocks represent Archean accretionary complexes with ages of ~3.5 Ga and 3.7–3.8 Ga, respectively. We analyzed the trace element contents including rare earth elements (REEs), and Sr and Nd isotopic compositions of the basalts, which may represent MORBs and OIBs, from NP and ISB. Their trace-element compositions are broadly similar, but show distinct geochemical characteristics particularly with respect to REEs that probably reflect differences in both the source mantle and degree of melting. Such differences are also evident in their initial Nd isotopic compositions, which were estimated based on equilibrium partitioning of REEs and well-defined isochron ages. In contrast, the Sr isotopic compositions of the NP and ISB basalts are highly variable and their isochron ages are inconsistent with previous studies. Furthermore, the partitioning of Rb and Sr in the NP basalts indicates disequilibrium, suggesting that the Rb-Sr system has been disturbed by post-igneous alteration and metamorphism. Based on these observations, we propose the following model to explain the temporal variations in the geochemical composition of the Archean mantle: (i) ~3800 Ma: recycling of plate material and melting occurred quite readily and, therefore, MORBs and OIBs were produced from differentiated mantle sources; (ii) 3460 Ma to ~3800 Ma: mantle-crust mixing occurred as the result of an extreme event, such as mantle overturning, reducing the compositional variation of the mantle; and (iii) after ~3460 Ma: mantle heterogeneity gradually developed in the material-recycling system, re-establishing the compositional differences between MORBs and OIBs. This model requires an extreme event to drive the homogenization during stage (ii), which may provide new insights into the evolution of the crust-mantle system.

中文翻译：

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从澳大利亚北极玄武岩和格陵兰岛上壳带的 Sr 和 Nd 同位素系统估计太古代地幔的成分非均质性

摘要 现代洋中脊玄武岩 (MORB) 和海岛玄武岩 (OIB) 中发现的成分变异反映了与地幔-地壳系统中物质循环相关的分化过程。为了研究这个循环系统建立的时间以及它如何转变为今天的系统，我们对来自西澳大利亚皮尔巴拉克拉通东皮尔巴拉克拉通的北极 (NP) 和 Isua 上地壳带的太古代玄武岩进行了地球化学分析(ISB)，西格陵兰岛南部。这些岩石代表了年龄分别为~3.5 Ga 和 3.7-3.8 Ga 的太古代增生杂岩。我们从 NP 和 ISB 分析了包括稀土元素 (REEs) 在内的微量元素含量，以及玄武岩的 Sr 和 Nd 同位素组成，它们可能代表 MORBs 和 OIBs。它们的微量元素组成大致相似，但显示出明显的地球化学特征，特别是在 REE 方面，可能反映了源地幔和熔融程度的差异。这种差异在它们的初始 Nd 同位素组成中也很明显，这是根据 REE 的平衡分配和明确定义的等时线年龄估计的。相比之下，NP 和 ISB 玄武岩的 Sr 同位素组成变化很大，它们的等时线年龄与以前的研究不一致。此外，NP 玄武岩中 Rb 和 Sr 的分配表明不平衡，表明 Rb-Sr 系统受到了火成岩后蚀变和变质作用的干扰。基于这些观察，我们提出以下模型来解释太古代地幔地球化学成分的时间变化：（i）~3800 Ma：板块材料的回收和熔化很容易发生，因此，MORBs 和 OIBs 是由不同的地幔来源产生的；(ii) 3460 Ma~3800 Ma：地幔-地壳混合是地幔翻转等极端事件的结果，降低了地幔成分的变化；（iii）~3460 Ma之后：地幔异质性在物质循环系统中逐渐发展，重新建立了MORBs和OIBs之间的成分差异。该模型需要一个极端事件来驱动阶段（ii）的均质化，这可能为壳幔系统的演化提供新的见解。如地幔翻转，减少地幔成分的变化；（iii）~3460 Ma之后：地幔异质性在物质循环系统中逐渐发展，重新建立了MORBs和OIBs之间的成分差异。该模型需要一个极端事件来驱动阶段（ii）的均质化，这可能为壳幔系统的演化提供新的见解。如地幔翻转，减少地幔成分的变化；（iii）~3460 Ma之后：地幔异质性在物质循环系统中逐渐发展，重新建立了MORBs和OIBs之间的成分差异。该模型需要一个极端事件来驱动阶段（ii）的均质化，这可能为壳幔系统的演化提供新的见解。